Search

The purpose of this study is the efficiency improvement of the Sb2Se3 solar cells. In this study, an experimental Sb2Se3 solar cell (glass/Mo/MoSe2/Sb2Se3/CdS/ZnO/ZnO:Al/Ag) with an efficiency record of 9.2% has been simulated. Absorber/buffer interface engineering plays a significant role in enhancing the efficiency of Sb2Se3 solar cells. To achieve this purpose, two approaches have been considered: first, adding sulfur to Sb2Se3 absorber, and second, using an alternative buffer with a wider bandgap instead of conventional CdS buffer. The effects of various x = Se/(S + Se) ratios of Sb2(S1−xSex)3 absorber layer on the photovoltaic performance were investigated. For Sb2(S,Se)3/CdS solar cell, optimum Se/(S + Se) mole fraction of 0.6 < x < 0.8 leads to improved efficiency. Also, ZnO1−ySy buffer layer was applied to replace the conventional CdS buffer layer of Sb2Se3 solar cells to reduce parasitic absorption and improve the short-circuit current density (Jsc). Bandgap for ZnO1−ySy semiconductor is higher than CdS. So, this leads to improved external quantum efficiency at short wavelengths. Optimization of band alignment through ZnO1−ySy buffer layer reduces interface carrier recombination and improves open-circuit voltage (Voc). The band alignment at the Sb2(S1−xSex)3/ZnO1−ySy interface is optimized by adjusting the selenium-to-sulfur ratio in the absorber layer and sulfur-to-oxygen ratio in the buffer layer. This work reveals that the most suitable interface for Sb2(S1−xSex)3/ZnO1−ySy heterojunction is formed when sulfur mole fractions ranging 0.5–0.6 in the Zn(O,S) buffer layer and 0.1–0.2 in the Sb2(S,Se)3 absorber layer. The results show that the efficiency improves from 9.2% to 15.65%, which represents a 70% improvement compared with the conventional Sb2Se3/CdS solar cell.

Global economies are transitioning towards net-zero emissions, but technological leaps are still needed to accelerate decarbonisation within the energy sector and beyond. Here, several novel material systems are studied to uncover physical properties which will dictate their suitability for use in state-of-the-art light-harvesting structures such as thin-film photovoltaics and photoelectrochemical fuel cells. Such materials offer promising avenues to cheap and efficient sustainable energy solutions. Metal halide perovskites excel in the pursuit of highly efficient thin film photovoltaics and light emitters. Substitution of the monovalent cations has advanced luminescence yields and device efficiencies. In this thesis, the change in photocarrier recombination behaviour caused by cation alloying is explored. Time-resolved optical spectroscopy and microscopy are used to reveal local charge accumulation in mixed cation perovskites, creating p- and n-type photodoped regions, unearthing a strategy for efficient light emission at low charge-injection in solar cells and light-emitting diodes. Operational stability of perovskite solar cells remains a barrier to their commercialisation, yet a fundamental understanding of degradation processes, including the specific sites at which failure mechanisms occur, is lacking. Here, multimodal microscopy techniques are utilised to show that nanoscale defect clusters, which are associated with phase impurities, are sites at which material degradation seeds. The trapping of charge carriers at sites associated with phase impurities, itself reducing performance, catalyses redox reactions that compromise device longevity. Importantly, this reveals that both performance losses and intrinsic degradation can be mitigated by eliminating these defective clusters. Carbon nanodots are an emergent material whose ease of fabrication and water solubility make them exciting candidates for photocatalytic processes. However, a full understanding of their excited charge carrier dynamics and interaction with common electron donors/acceptors is not yet established. This work identifies charge transfer processes in hybrid photocatalytic systems with carbon nanodot absorbers and builds bottom-up mechanistic insight., The author acknowledges support from the Engineering and Physical Sciences Research Council (EPSRC) and the Japan Society for the Promotion of Science (JSPS)

Potential barriers between the waveguide layer and MQW active region may influence injection efficiency significantly, which is important in improving output characteristics of GaN-based green laser diodes (LDs). In this study, potential barriers and injection efficiency of LDs are investigated by simulation methods. It is found that different indium content in quantum barrier layers results in different potential barrier heights, leading to different recombination rates in upper and lower waveguide layers, and the injection efficiency can be modulated effectively. An eclectic choice of indium content can suppress recombination in two waveguide layers, improving the output characteristics of green LDs. Additionally, a composite lower waveguide layer structure is proposed to reduce the negative effect of potential barriers. High output power and low threshold current are achieved owing to the reduction in electron injection blockage and hole leakage effects.

Electroluminescence of LED heterostructures with active layer made of InAsSb films grown on InAs substrates was studied in the temperature range T = 4.2–300 K. At low temperatures (T = 4.2–100 K), stimulated emission was observed with an optical cavity formed normal to the growth plane. The emission became spontaneous at higher temperatures due to the resonant “switch-on” of the CHHS Auger recombination process in which the energy of a recombining electron-hole pair is transferred to a hole transitioning to the spin-orbit-splitted band. The spontaneous character of emission continued up to room temperature because of the influence of other Auger processes. The results obtained suggest that InAsSb-based LED heterostructures are promising for the fabrication of vertically-emitting mid-infrared lasers.

Metal halide perovskites, of general formula ABX3, show a complex interplay of the inorganic BX3 sub-lattice and the organic/inorganic A-site cations, which likely determines some of their peculiar optoelectronic properties. Comprehension of the physics underlying this interaction may reveal further means of fine-tuning their optoelectronic response. Here, we investigate in depth charge/lattice interactions associated to the formation of polarons in different models of the prototypical CH3NH3PbI3 perovskite through advanced electronic-structure calculations. We demonstrate that charge localization, while induced by the disordered dipolar field of the organic cations, is only stabilized by distortions in the inorganic sub-lattice. Polaron hopping between neighbouring minima is related to the random reorientation of the organic cations and occurs via a delocalized transition state. Our simulations highlight a struggle between thermally induced disorder, driven by the motion of A-site cations, and polaron stabilization within the BX3 sub-lattice, which explains the simultaneous low mobility and high diffusion length of charge carriers in lead-halide perovskites.

Carrier recombination and scattering at the semiconductor boundaries can substantially limit the device efficiency. However, surface and interface recombination is generally neglected in the nitride-based devices. Here, we study carrier recombination and diffusivity in AlGaN/GaN/sapphire heterointerfaces with AlGaN barriers of different quality. We employ the light induced transient grating and time-resolved photoluminescence spectroscopy techniques to extract carrier lifetime in different depths of the GaN buffer as well as in the AlGaN barrier, and to evaluate the carrier diffusion coefficient in the buffer. Moreover, we assess interface recombination velocity, Shockley-Read-Hall and radiative recombination rates. We reveal the adverse barrier influence on carrier dynamics in the underlying buffer: AlGaN barrier accelerates the nonradiative carrier recombination in the GaN buffer. The interface recombination velocity in the GaN buffer increases with decreasing AlGaN barrier quality, and the dominating recombination mechanism switches from Shockley-Read-Hall to interface recombination. These phenomena are governed by a cumulative effect of various interface-deteriorating barrier defects. Meanwhile, the carrier diffusivity in the GaN buffer is not affected by the AlGaN barrier. We conclude that barrier-accelerated interface recombination can become a major carrier loss mechanism in AlGaN/GaN interface, and may substantially limit the efficiency in nitride-based UV LEDs.

Deployment of photovoltaic power generation is expected to accelerate over the next 10-20 years under the influence of reduced cost and increased power conversion efficiency. An important limiting factor to cell efficiency is carrier recombination at metal contacts, thus, to aid further improvements, an accurate and reliable measurement technique for this recombination is required. This recombination is challenging to measure in isolation from other sources because measurements at the cell terminals convolve many types of recombination into a single measurement, and simplified test structures are not easily measured using traditional photoconductance based techniques for surface recombination measurement because the metal interferes with the conductance measurement. Photoluminescence (PL) measurements are ideally suited to studying recombination of metallised cells and test structures because they are contactless, applicable to arbitrarily size areas and minimally influenced by the presence of metal. Following other studies which apply PL imaging to this problem, this thesis investigates dynamic PL measurement techniques which are faster, simpler and do not require external calibration. A dynamic PL measurement system is first developed and tested. The system is then used for detailed investigation of silicon-metal interface recombination. A detailed study comparing dynamic open circuit voltage measurements (called Suns-Voc) and implied open circuit voltage measurements obtained from the developed PL system (called Suns-PL) shows that Suns-Voc data fail to accurately represent all of the recombination in a device at high illumination conditions due to lateral resistive effects but Suns-PL is unaffected by this effect and thus is well suited to measuring metallised cells and test structures. Metal contact recombination measurements using dynamic PL and the subsequent data analysis are also investigated. It is concluded that because of the non-uniform recombination introduced by metal contacts, excess carriers tend to become non-uniformly distributed, and analysis techniques that assume uniform distribution are inaccurate. Analysis techniques that are based on the simulating the full device geometry such as the methods presented in this thesis can better account for the non-uniformity and are thus more accurate.

Sub-band-gap (SBG) impact ionization (II) enables steep subthreshold slope that enables devices to overcome the thermal limit of 60mV/decade. This phenomenon at low voltage enables various applications in logic, memory and neuromorphic engineering. Recently, we have demonstrated sub-0.2V II in NIPIN diode experimentally primarily based on the steady-state analysis. In this paper, we present the detailed experimental transient behavior of SBG-II in NIPIN. The SBG-II generated holes are stored in the p-well. First, we extract the leakage mechanism from the p-well to show two mechanisms (i) recombination-generation (RG) and (ii) over the barrier (OTB) where OTB dominates when barrier height $ phy_b, Comment: 6 pages, 9 figures, journal IEEE TED

Hybrid silicon/organic solar cells have been recently extensively investigated due to their simple structure and low-cost fabrication process. However, the efficiency of the solar cells is greatly limited by the barrier height as well as the carrier recombination at the silicon/organic interface. In this work, hydrochloroplatinic acid (H2PtCl6) is employed into the poly(3,4-ethlenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution, and the work function (WF) of the PEDOT:PSS layer has been successfully improved. Based on the Pt-modified PEDOT:PSS layer, the efficiency of the silicon/PEDOT:PSS cell can be increased to 11.46%, corresponding to ~20% enhancement to the one without platinum (Pt) modification. Theoretical and experimental results show that, when increasing the WF of the PEDO:PSS layer, the barrier height between the silicon/PEDOT:PSS interface can be effectively enhanced. Meanwhile, the carrier recombination at the interface is significantly reduced. These results can contribute to better understanding of the interfacial mechanism of silicon/PEDOT:PSS interface, and further improving the device performance of silicon/organic solar cells. Electronic supplementary material The online version of this article (doi:10.1186/s11671-016-1759-0) contains supplementary material, which is available to authorized users.

Carrier recombination is a central process in bulk heterojunction organic solar cells. Based on the competition of hopping rates that either implies escape in a broad density of states or recombination across the interface, we formulate a general theory of recombination flux that distinguishes reaction or transport limited recombination according to charge density. The Langevin picture is valid only in the low charge density limit, and a crossover to the reaction controlled regime occurs at higher densities. We present results from impedance spectroscopy of poly(3-hexylthiophene):methanofullerene solar cell that exhibit this crossover The work was supported by Generalitat Valenciana (Project No. ISIC/2012/008).

In order to improve the electrical performances of Cu2ZnSnSe4 (CZTSe) based solar cells, the standard CdS buffer layer was replaced by (Cd,Zn)S processed by chemical bath deposition. The morphology and composition of the (Cd,Zn)S thin films were studied as a function of [Zn]/([Zn] + [Cd]) ratio in the chemical bath (80, 85 and 90%). The CZTSe/(Cd,Zn)S solar cells with and without Cd partial electrolyte (Cd PE) treatment were compared to CZTSe/CdS reference devices using current–voltage and external quantum efficiency measurements. The (Cd,Zn)S thin films show a non-homogeneity of Zn distribution and phase formation, with a shift from Zn(O,OH)x to ZnS phase when increasing the deposition time and a decrease of the layers thicknesses when increasing the Zn concentration in chemical bath. A model for the growth of (Cd,Zn)S thin films is proposed. The resulting CZTSe/(Cd,Zn)S devices show an important reduction of the barrier at the hetero-interface, which is attributed to the lower density of O contamination in (Cd,Zn)S compared to CdS, inducing a lower density of deep p-type recombination centers. Despite the reduced compensation of the buffer layer, CZTSe/(Cd,Zn)S devices show a deterioration of the open circuit voltage and the fill factor with the increase of Zn content in (Cd,Zn)S. These electrical losses were avoided by Cd PE treatment prior to the deposition of (Cd,Zn)S. This work was supported in part by the Flemish government, Department of Economy, Science and Innovation (EWI). Tom De Geyter, Greetje Godiers and Guido Huyberechts from Flamac in Gent are acknowledged for sputtering of the metal layers. AGC is acknowledged for providing substrates.

We characterize 5.5 MeV alpha particles emitted from 241 Am with a liquid-phase epitaxially grown InSb crystal detector at operating temperatures ranging from 5 to 104 K. The pulse height of the energy peak of alpha particles rapidly decreases at temperatures above 40 K. We explain this behavior of the peak channel numbers as a function of temperature using a model of the recombination and trapping of generated electrons and holes.

Date du colloque: 12/2009; International audience; We have investigated electrical, charge transport and optical properties of functionalised DNA:PEDT-PSS thin films. Current-voltage dependencies of the samples were linear and symmetrical down to 77 K temperature. Material conductivity at room temperature was about (1-5)×10-10 ¿-1 cm-1. The thermal activation energy of the conductivity measured in the dark was about 0.033 eV near the room temperature independently on the applied bias. The weakly expressed carrier trapping was identified by the thermally stimulated current method, evidencing the fast recombination of light-generated carriers. Though, at constant light excitation a "bistable" photoconduction below the room temperature was identified. I.e., upon excitation by light from the spectral region 500 800 nm a notable increase of the photocurrent could be observed below 140 160 K by cooling the samples. Meanwhile by heating the photosensitivity remained increased up to 230 240 K. Most probably such phenomenon could be attributed to the light-induced morphology changes of the samples.

Epitaxially-grown three-dimensional Si/SiGe nanostructures (NSs) produce photoluminescence (PL) and electroluminescence in the desired spectral range of 1.3-1.6 μm. We show that by controlling and modifying such Ge-rich SiGe nanoclusters during growth it is possible to fabricate very fast (PL lifetime, 223rd ECS Meeting, Nanocrystal Embedded Dielectrics for Electronic and Photonic Devices, May 12-16, 2013, Toronto, ON, Canada, Series: ECS Transactions

This paper discusses the modeling of the measurements which are performed with the charge-collection, or Electron Beam Induced Current (EBIC) technique of the scanning electron microscope, and the use of the related theoretical results for recovering bulk and local recombination properties of semi conductors. A general description of different EBIC measurements can be given on the basis of the notion of charge-collection probability 𝝋(r) in the device being examined. This function can be calculated by solving a stationary diffusion equation and the induced current results from the convolution of 𝝋(r) with the generation function of the electron beam. According to this approach, EBIC experiments give information about 𝝋 or the essential semiconductor or defect parameters upon which 𝝋 is dependent. The more usual procedures to recover this information from actual measurements are reviewed and some new possibilities are examined.

Electron beam induced current (EBIC) microscopy is a very promising SEM technique for the study of diffusion length and depletion region width inhomogeneities with high spatial resolution. However, this resolution is limited by the dimensions of the electron-hole pair generation region. In this paper the possibilities to improve spatial resolution are discussed. Electron beam tomography, i.e. the reconstruction of physical property distributions from sets of two-dimensional images, seems to be the most promising for this purpose. It is shown that in the case of dislocations it is possible to obtain information about dislocation impurity atmosphere parameters from EBIC measurements. The spatial resolution limitations in SEM techniques are discussed and it is shown that for many structures the spatial resolution is restricted by electron beam damage.